Colored optical fiber and optical fiber ribbon assembly containing said fiber

ABSTRACT

Optical fiber ribbon having an optical fiber with a radiation curable internal coating and a radiation curable colored coating disposed to surround the internal coating, and a radiation curable matrix material surrounding one or more of the optical fibers to form a ribbon, in which: the colored coating has a degree of adhesion to the inner coating which is higher than the degree of adhesion to the matrix material; and the optical fiber in the optical fiber ribbon shows, upon aging of the ribbon for at least two weeks in water at 60° C., an increase in the attenuation of the transmitted signal at 1550 nm of less than 0.05 db/km with respect to the attenuation of the assembled optical fiber measured before aging.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase application based on internationalapplication number PCT/EP01/06769, filed on Jun. 15, 2001, and claimsthe benefit of U.S. Provisional Patent Application No. 60/213,237, filedon Jun. 22, 2000.

The invention was developed under a joint research agreement betweenPirelli Cavi S.p.A. and DSM Desotech B.V.

BACKGROUND OF THE INVENTION

The invention relates to an optical fiber comprising an internal coatingand a colored coating, further called colored optical fiber, and to anoptical fiber ribbon comprising a plurality of said colored opticalfibers.

Optical glass fibers are generally coated with two superposedradiation-cured coating layers, which together form the so-calledprimary coating or primary coating system. The coating layer (morebriefly “coating”) which is in direct contact with the glass is calledthe inner primary coating and the overlaying coating, which is on theexposed surface of the coated fiber, is called the outer primarycoating. The inner primary coating may also be called the primarycoating; then, the outer primary coating is called the secondarycoating. Both definitions are used interchangeably.

The inner primary coating is usually a relatively soft material whilethe outer primary coating is a relatively harder material. The primarycoating system is designed to provide environmental protection to theglass fiber and resistance, inter alia, to the well-known phenomenon ofmicrobending, which can lead to attenuation of the signal transmissioncapability of the fiber and is therefore undesirable. In addition, theprimary coating system is designed to provide the desired resistance tophysical handling forces, such as those encountered when the fiber issubmitted to cabling operations.

In general, the primary coating system is applied onto the optical fiberduring the drawing manufacturing process of the optical fiber.

In telecommunications applications of optical fibers, multipleindividual strands of coated fiber can be packaged into largerstructures such as ribbons and cables, to maximize efficiency. However,after ribboning and cabling of fiber, the individual strands of fibermust be readily distinguishable from each other so they can beaccurately identified during, for example, installation and repair.Cable geometry and/or color coding can be used to distinguish andidentify individual fibers in a complex cable.

Although several methods can be used to color code fiber, color codingcan be done advantageously with either a thin colored layer (about 10microns or less), also called an ink composition, which is placed overthe primary coated fiber before cabling and/or ribboning of the same orby applying a colored outer primary coating onto the inner primarycoating.

Typically, the application of the colored outer primary coating onto theinner primary coating takes place during the drawing process of theoptical fiber. On the other side, the application of a colored layeronto the primary coated optical fiber generally takes place on aseparate manufacturing line, after the primary coated optical fiber hasbeen produced.

For the sake of conciseness, in the following of the presentspecification the term “internal coating” will indicate a coatingdisposed to surround the glass portion of the optical fiber, thuscomprising either an “inner primary coating” or a “primary coatingsystem” (i.e. comprised of an inner and an outer primary coating). Saidinternal coating is then in turn coated with a colored coating. Theterms “colored coating composition”, “colored layer”, “ink layer” and“ink composition” are used interchangeably throughout the specification.

Tape-like optical fiber ribbons are prepared by embedding at least twoindividual color coded fibers in a supporting matrix material which,like the inner and outer primary coatings, is also radiation-curable tomaximize production speed. Optical fiber ribbons may comprise, forinstance, 4 to 12 colored fibers. The matrix material can encase thecolor coded optical glass fiber or the matrix material can edge-bond theglass fibers together. Cure of the matrix material occurs during theribboning stage after the fibers have been color-coded by applying acolored coating. Hence, in a ribbon design, the ink layer residesbetween the ribbon's matrix material and the fibers' outer primarycoating.

This means that the ink layer's interfacial characteristics (e.g.,surface energy, adhesion) must be carefully controlled to functionproperly with both matrix material and outer primary coating in theribbon structure. In particular, the ability of a cured matrix materialto be suitably stripped off the ink layer (break-out) is an importanttechnical consideration. Ribbon break-out is generally carried out by amechanical force, although chemical softening of the matrix with use ofsolvents is also known.

Optical fiber color coding can be based on up to 12 or more colors.Although optical fiber inks were originally solvent-based orthermosetting inks, in more recent times, radiation-curable inks havebeen used to increase the speed of the inking process. In these inkcompositions, pigment is dispersed in a radiation-curable carrier orbase composition.

As the demand for coated optical glass fibers has increased,manufacturers must respond by adding more fiber drawing production linesand by attempting to increase the linear line speeds of the existingfiber drawing/coloring production lines. In the latter case, one factorwhich will determine the upper limit for the line speed will be thecuring rate characteristics of the radiation-curable ink composition,for a given radiation source and intensity.

If the line speed is increased to the extent that cure rate timerequirements of the radiation curable ink composition are not provided,the radiation curable ink composition will not have received asufficient amount of radiation to cause complete cure, or cross-linking,of the radiation-curable ink composition.

The production linear line speed is generally inversely related to theamount of radiation striking the optical glass fiber. That is, as theproduction line speed is increased, the amount of radiation exposure tothe radiation-curable ink composition during the production process willnecessarily decrease for a given radiation source. Incomplete cure ofthe radiation-curable ink composition is undesirable and must be avoidedbecause then the desired properties of the ink coating may not beachieved and/or the incompletely cured ink coating may retain tackiness(giving problems in subsequent handling) or a malodorous odor may bepresent, and there may also be an undesirable increase of extractablecomponents in the supposedly-cured ink coating.

In general, radiation-curable ink coating compositions cure at asignificantly slower rate than radiation-curable outer primary coatingcompositions.

It is believed that the pigments present in ink compositions contributeto the slower cure speed of ink coatings. Thus, there is a need forimproving the cure speed of the ink.

While the ink composition must have a very fast cure speed to ensurecomplete cure of the ink coating on the high speed drawing/coloringlines, the increase in cure speed should not come at the expense ofother important properties of the ink coating, such as that providingsuitable break-out performance. Break-out performance is the ability ofthe cured ink coating to separate from the matrix material withoutseparating the ink layer from the outer primary coating, to provide aneasy access to the individual coated optical glass fibers containedwithin the ribbon assembly, for instance during cabling/connectionoperations of the optical fibers.

Therefore, a radiation-curable ink composition should preferably exhibitadaptable adhesion properties to provide an adhesion between the outerprimary coating and the ink coating that is greater than the adhesionbetween the ink coating and the matrix material to provide easy fiberaccess.

International Patent application Publication No. WO 98/50317 discloses aribbon assembly comprising a colored optical fiber, wherein the coloredcoating of said optical fiber is formed from a radiation curable systemwhich contains a mixture of oligomers, monomers and at least onephotoinitiator, selected in such a way as to provide a level of adhesionbetween the ink coating and the matrix material which is less than thelevel of adhesion between said ink coating and the underlying innercoating of the optical fiber.

Patent application EP-A-614099 describes the use of a release agent suchas a silicon oil or a fluororesin between the bundling layer and thecoloring layer. In particular, when substantial amounts of siliconeresins are used, incompatibility in the liquid and imperfections in thecured matrix composition may result, which causes attenuation of light.

Published Japanese patent application JP-A-01022976 describes aradiation curable ink composition comprising an alkoxylated bisphenol Adiacrylate oligomer, a trifunctional reactive diluent and a homolyticphotoinitiator.

SUMMARY OF THE INVENTION

The Applicant has now observed that, while some of the known inkcompositions may satisfy the above different adhesion requirements,these inks generally have an insufficient resistance to water, inparticular when a ribbon comprising the coated and inked optical fibersis soaked in water for a relatively long period of time. Thischaracteristic is further called in the present specification the “watersoak resistance” of a fiber. Other of the known ink compositions, whichmay show the desired water soak resistance do not however fulfill theadhesion requirements

In the present application, water soak resistance is referred to thecapability of the fiber to maintain substantially unaltered its opticaland mechanical parameters upon exposure to water. This property canadvantageously be determined by measuring the variation of theattenuation value of the signal transmitted through an optical fiberimmersed in water. In the following, when referring to the water soakproperties of an optical fiber, the term “optical fiber” includes withinits meaning either an optical fiber as such or an optical fiber disposedwithin a matrix material to form a ribbon of fibers. According to whatobserved by the Applicant, fibers having good water soak properties arethose wherein the attenuation value is substantially constant in timewhen the fiber is immersed in water at a predetermined temperature andfor a predetermined time.

In particular, the variation of the measured attenuation value should beless than about 0.05 db/km for at least two weeks when the fiber isimmersed into water at a temperature of 60° C. As a matter of fact, asobserved by the Applicant, fibers showing an increase of more than 0.05db/km within less than two weeks of testing can not guarantee reliableoptical performances during their entire operating life.

Although not wishing to be bound by any particular theory, it isbelieved that the increase in the attenuation value of an optical fiberimmersed in water can be correlated to the fact that water may penetrateat the interface between two coating layers, thus determining possiblemicrobending phenomena which may cause an increase in the attenuation ofthe transmitted signal.

The Applicant has further observed that while a fiber coated with acolored layer may show good water soak performances when tested as asingle fiber, the same fiber may have unacceptable properties whencoated with a matrix material to form an optical fiber ribbon. Asobserved by the Applicant, the interface between the colored layer andthe matrix layer is thus the most critical interface for the water soakproperties of optical fiber ribbons. Therefore, a relative good adhesionbetween the colored layer and the matrix layer should be achieved andmaintained during the entire operating life of the optical fiber, inorder to avoid water penetration at the interface of these two layers.

There is thus an apparent incompatibility between the requirement ofgood release properties and the requirement of good water soakproperties. While the first property requires a relatively low degree ofadhesion between the colored layer and the matrix, the second propertyrequires a rather good adhesion between the two layers, which would notbe affected by decay due to water presence.

Having recognized the above problem, the Applicant has now found that itis possible to optimize the release properties and the water soakproperties of the optical fiber, in particular when said optical fiberis disposed within an optical fiber ribbon, by suitably formulating thecomposition of the resin which is applied as the colored coating inorder to achieve acceptable values of both these properties.

One aspect of the present invention thus relates to an optical fibercomprising an internal coating and a colored coating disposed tosurround said internal coating wherein, when said fiber is coated with amatrix material and assembled into an optical fiber ribbon:

-   -   said colored coating has a degree of adhesion to the inner        coating which is higher than the degree of adhesion to the        matrix material; and    -   said optical fiber assembled into said optical fiber ribbon        shows, upon aging for at least two weeks in water at 60° C., an        increase in the attenuation of the transmitted signal at 1550 nm        of less than 0.05 db/km with respect to the attenuation of the        assembled optical fiber measured before aging.

A further aspect of the present invention relates to an optical fiberribbon comprising a plurality of optical fibers bound together by amatrix material, said fibers comprising an internal coating and acolored coating disposed to surround said internal coating, wherein saidcolored coating has a degree of adhesion to the internal coating whichis higher than the degree of adhesion to the matrix material, saiddegree of adhesion to the matrix material being however sufficientlyhigh such that said optical fibers show, upon aging for at least twoweeks in water at 60° C., an increase in the attenuation of thetransmitted signal at 1550 nm of less than 0.05 db/km with respect tothe attenuation of the optical fibers measured before aging.

Preferably, the increase in the attenuation of the transmitted signal at1550 nm is less than about 0.05 db/km, upon aging of the assembled fiberfor at least one month in water at 60° C. More preferably, the fiber isaged in water at 60° C. for at least two months without showing saidattenuation's increase, particularly preferred being an ageing of atleast four months without showing said attenuation's increase.

Preferably, said internal coating comprises an inner primary coating andan outer primary coating and the colored coating has a thickness of fromabout 3 to about 10 microns.

A further aspect of the present invention relates to an optical fibercomprising a radiation curable internal coating and a radiation curablecolored coating disposed to surround said internal coating wherein saidcolored coating comprises

(A) 40–60% by weight of a bisphenol A epoxy diacrylate, a modifiedbisphenol A epoxy diacrylate or a mixture of both,

(B1) 15–30% by weight of an alkoxylated aliphatic glycol diacrylatediluent,

(B2) 5–25% by weight of trifunctional acrylate diluent,

(C) 6–20% by weight of a photoinitiator system consisting of less than4% by weight of benzophenone and at least two different homolyticfree-radical photoinitiators,

(D) 1–9% by weight of a polydimethylsiloxane based silicone releaseagent; and

(E) 1–15% by weight of a dry pigment;

wherein said composition comprises less than 5% by weight of a urethanebased acrylate,

whereby, if said fiber is coated with a radiation curable matrixmaterial and assembled into an optical fiber ribbon, said optical fibershows, upon aging for at least two weeks in water at 60° C., an increasein the attenuation of the transmitted signal at 1550 nm of less than0.05 db/km with respect to the attenuation of the assembled opticalfiber measured before aging.

Preferably, the two homolytic photoinitiators of component (C) differ intheir respective photosensitivity.

Preferably, said radiation curable colored coating composition furthercomprises less than 3% by weight of N-vinyl caprolactam.

Preferably, said radiation curable colored coating composition comprisesas the trifunctional acrylate diluent (B2) trimethylol propanetriacrylate.

According to a particularly preferred embodiment, said radiation curablecolored coating composition consists essentially of:

(A) 40–60% by weight of a bisphenol A epoxy diacrylate, a modifiedbisphenol A epoxy diacrylate or a mixture of both,

(B1)15–30% by weight of an alkoxylated aliphatic glycol diacrylatediluent,

(B2) 5–25% by weight of trimethylol propane triacrylate,

(C) 6–20% by weight of a photoinitiator system consisting of less than4% by weight of benzophenone and at least two homolytic free-radicalphotoinitiators,

(D) 1–9% by weight of a polydimethylsiloxane based silicone releaseagent, and

(E) 1–15% by weight of a dry pigment.

Preferably the above mentioned alkoxylated aliphatic glycol diacrylatediluent (B1) is ethoxylated aliphatic glycol diacrylate.

Preferably the above mentioned component (D) is a non-reactivepolydimethyl siloxane based silicone release agent.

DETAILED DESCRIPTION OF THE INVENTION

Radiation-curable carrier systems which are suitable for forming an inkcomposition to be used in an optical fiber according to the inventioncontain one or more radiation-curable oligomers or monomers having atleast one functional group capable of polymerization when exposed toactinic radiation. Suitable radiation-curable oligomers or monomers arenow well known and within the skill of the art. Commonly, theradiation-curable functionality used is ethylenic unsaturation, whichcan be polymerized preferably through radical polymerization.Preferably, at least about 80 mole %, more preferably, at least about 90mole %, and most preferably substantially all of the radiation-curablefunctional groups present in the oligomer are acrylate or methacrylate.For the sake of simplicity, the term “acrylate” as used throughout thepresent application covers both acrylate and methacrylate functionality.

A suitable radiation-curable ink composition essentially consists offrom about 1 to about 80 weight % of at least one radiation curableoligomer (A). Preferred amounts of the radiation curable oligomerinclude from about 20 to about 70% by weight, based on the total weightof the ink composition.

A mixture of mono-, di-, tri-, tetra-, and higher functionalizedoligomers can be used to achieve the desired balance of properties,wherein the functionalization refers to the number of radiation curablefunctional groups present in the oligomer. The oligomers usuallycomprise a carbon-containing backbone structure to which the radiationcurable functional group(s) are bound.

Examples of suitable carbon-containing backbones include polyethers,polyolefins, polyesters, polyamides, polycarbonates and polyacrylates.The size of the carbon-containing backbone can be selected to providethe desired molecular weight. The number average molecular weight of theoligomer is usually between about 500 to about 10,000, preferablybetween about 500 to about 7,000, and most preferably between about1,000 to about 5,000.

For example, the carbon-containing backbone of the oligomer can comprisearomatic groups and ring-opened epoxy groups or alkoxy groups. Theoligomer can be represented by, for example:

-   -   R—Ar—R; or    -   R—L—Ar—L—R

where R is a radiation-curable functional group, Ar is an aromatic groupcontaining moiety, and L is a linking group.

Examples of suitable linking groups include alkoxy or ring opened epoxysuch as ethoxy, propoxy, butoxy, and repeat units thereof. L can also bea urethane or urea linking group, but preferably there is substantiallyno urethane or urea group present, in particular, less than about 5% byweight, more preferably less than about 3% by weight.

The aromatic groups can be, for example, derived from bisphenol units,such as bisphenol A or bisphenol F.

A preferred oligomer is a diglycidyl ether derivative of bisphenol A towhich acrylate functional groups have been bound.

In a preferred embodiment of the present invention, theradiation-curable oligomer (A) according to the present invention is anyoligomer comprising an ethylenically unsaturated group which issubstantially free of urethane acrylates and is rich in epoxy acrylates.Preferably, the oligomer is a bisphenol A epoxy diacrylate.

The amount of oligomer is preferably from about 30% by weight and 70% byweight, more preferably from about 40% by weight to about 60% by weightbased on the total weight of the colored photo-curable composition.

Further to the above indicated conventional oligomers, the Applicant hasfound that particularly valuable adhesion properties of the coloredlayer can be obtained when said colored layer comprises a modifiedbisphenol A epoxy diacrylate, said modification being capable ofincreasing the hydrophobicity and/or adhesion characteristics of thecolored coating to the internal coating. Said modified bisphenol A epoxydiacrylate is preferably a fatty acid modified bisphenol A epoxydiacrylate.

According to a particularly preferred embodiment, the oligomer of saidcolored composition is a mixture of bisphenol A epoxy diacrylate and ofmodified bisphenol A epoxy diacrylate, the ratio between the unmodifiedand modified bisphenol A epoxy diacrylate being from about 0.8:1 toabout 1:1.

According to another preferred embodiment, the ratio between unmodifiedand modified bisphenol A epoxy diacrylate is from about 2.4:1 to about2.2:1, more preferred, about 2.3:1, by which a colored layer having animproved balance of properties can be achieved. In particular, a coloredlayer having an improved MEK resistance can be achieved.

Commercially available example of (modified) bisphenol A epoxydiacrylate is Ebecryl 3700 (UCB) or CN-120 (Sartomer), the latter havinga molecular weight of about 1300, and when cured has a Tg of about 65°C. Modified bisphenol A epoxy diacrylates are available e.g. as Ebecryl3702 (UCB), having a Tg of about 56° C. when cured, and CN-116(Sartomer).

The radiation-curable carrier systems may also contain one or morereactive diluents (B) which are used to adjust the viscosity. Thereactive diluent can be a low viscosity monomer having at least onefunctional group capable of polymerization when exposed to actinicradiation. This functional group may be of the same nature as that usedin the radiation-curable oligomer. Preferably, the functional group ofeach reactive diluent is capable of copolymerizing with theradiation-curable functional group present on the otherradiation-curable diluents or oligomer. The reactive diluents used canbe mono- and/or multifunctional, preferably (meth)acrylate functional.

A suitable radiation-curable ink composition comprises from about 1 toabout 80% by weight of at least one radiation-curable diluent. Preferredamounts of the radiation-curable diluent include from about 10 to about60% by weight, more preferably from about 20 to about 55% by weight,based on the total weight of the ink composition.

Generally, each reactive diluent has a molecular weight of less thanabout 550 and a viscosity of less than about 500 mPas.

For example, the reactive diluent can be a monomer or a mixture ofmonomers having an acrylate or vinyl ether functionality and a C₄–C₂₀alkyl or polyether moiety. Preferably, there is substantially nomonoacrylate present, but there can be non-acrylate functional monomerdiluents present, which are capable of reacting with theradiation-curable functional group present on the radiation-curablemonomer or oligomer. Examples of such non-acrylate functional monomerdiluents are N-vinylpyrrolidone, N-vinyl caprolactam and the like.

These N-vinyl monomers preferably are present in amounts between about 1and about 20% by weight, more preferably less than about 10% by weight.

The reactive diluent can also comprise a diluent having two or morefunctional groups capable of polymerization. Examples of such monomersinclude:

C₂–C₁₈ hydrocarbon-diol diacrylates,

C₄–C₁₈ hydrocarbon divinylethers,

C₃–C₁₈ hydrocarbon triacrylates, and the polyether analogues thereof,and the like, such as

1,6-hexanediol diacrylate,

trimethylolpropane tri-acrylate,

hexanediol divinylether,

triethyleneglycol diacrylate,

pentaerythritol-triacrylate,

ethoxylated bisphenol-A diacrylate, and

tripropyleneglycol diacrylate.

Such multifunctional reactive diluents are preferably (meth)acrylatefunctional, preferably difunctional (component (B1)) and trifunctional(component (B2)).

Preferably, alkoxylated aliphatic polyacrylates are used, such asethoxylated hexanedioldiacrylate, propoxylated glyceryl triacrylate orpropoxylated trimethylol -propane triacrylate.

Preferred examples of diacrylates are alkoxylated aliphatic glycoldiacrylate, more preferably, propoxylated aliphatic glycol diacrylate,particularly preferable, propoxylated neopentyl glycol diacrylate.

A preferred example of a triacrylate is trimethylol propane triacrylate.

Examples of higher functional reactive diluents are cited above.

The photoinitiators used in the ink composition of the present inventionpreferably are free-radical photoinitiators such as Norrish Type I andType II photoinitiators.

At least one of the photoinitiators (C) used in the ink coatingcomposition of the present invention is a homolytic fragmentationphotoinitiator (also called a Norrish Type I photoinitiator) whichoperates by intramolecular bond cleavage.

Examples of suitable Type I (homolytic) photoinitiators are benzoinderivatives, methylolbenzoin and 4-benzoyl-1,3-dioxolane derivatives,benzilketals, (α,α-dialkoxy-acetophenones, (α-hydroxy alkylphenones,(α-amino-alkylphenones, acylphosphine oxides, acylphosphine sulphides,o-acyl-α-oximinoketones, halogenated acetophenone derivatives, andbenzoyl diaryl phosphine oxides.

Commercial examples of suitable Type I photoinitiators are Darocur 1173(2-hydroxy-2-methyl-1-phenylpropane-1-one as the active component),Irgacure 184 (hydroxy-cyclohexyl phenyl ketone as the active component),Irgacure 907 (2-methyl-1-[4-methylthio)phenyll-2-morpholinopropan-1-one), Irgacure 369 (2-benzyl-2-dimethylamino-1(morpholinophenyl) -butanone-l as the active component), acylphosphinessuch as Lucirin TPO by BASF (2,4,6-trimethylbenzoyl -diphenyl -phosphineoxide) or Irgacure 1700 by Ciba Geigy(bis(2,6-dimethoxy-benzoyl)-2,4,4-trimethylpentyl phosphine oxide).

Also mixtures of Type I photoinitiators can be used.

Examples of suitable Type-II (hydrogen abstraction) photoinitiators arearomatic ketones such as benzophenone, xanthone, derivatives ofbenzophenone, Michler's ketone, thioxanthone and other xanthonederivatives like ITX (isopropyl thioxanthone), and the like. Chemicalderivatives and combinations of these photoinitiators can also be used.Preferably, benzophenone is present in an amount of less than about 4%by weight, more preferably, less than about 3% by weight, particularlypreferred less than about 2.5% by weight.

Type-II photoinitiators generally are used with an amine synergist.However, the ink composition according to the present invention containssubstantially no amine synergists, preferably in an amount of less thanabout 1% by weight, and more preferably less than about 0.1% by weight.

The radiation-curable ink composition of the present invention comprisesfrom about 6 to about 20% by weight of a photoinitiating system (C).Preferably, said photoinitiating system (C) comprises at least twohomolytic photoinitiators, more preferably three homolyticphotoinitiators. Preferably, the at least two homolytic photoinitiatorsof the photoinitiator system (C) differ in their respective photosensitivity.

Preferred amounts of the homolytic photoinitiator are from about 8 toabout 10% by weight, more preferred, from about 6 to about 8% by weight.

For an optimum cure speed in the presence of pigment, it is advantageousto combine an acyl phosphine oxide photoinitiator with one or more otherphotoinitiators, such as 2-methyl-1-[4-methylthio)phenyll-2-morpholinopropan-1-one and/or 2-hydroxy-2-methyl-1-phenylpropane-1-one.

Any inorganic and organic pigment (E) that is suitable for makingradiation-curable ink compositions can be used in the present invention.The preferred pigments are pigments that absorb light of a visiblewavelength, i.e. any color except pure white.

The use of the term “pigment” refers to both inorganic and organicpigments.

Preferably, the pigment used in the ink coating composition of thepresent invention is an organic pigment. The pigment can be present inthe ink composition in an amount that provides coloration that isvisible without magnification to facilitate identification of theindividual colored optical glass fiber.

Ribbon assemblies utilizing 12 or less coated optical glass fibersrequire only 12 colors to adequately distinguish each of the coatedoptical fibers from one another. However, in larger ribbon assemblies,more than 12 colors may be utilized to adequately distinguish the coatedoptical glass fibers from one another. Examples of twelve colorsnormally used for making ribbon assemblies include: black, white,yellow, blue, red, green, orange, brown, pink, aqua, violet, and gray.

Preferably, the pigment has a mean particle size of not more than about1 μm. The particle size of the commercial pigments can be lowered bymilling, if necessary.

A specific example of a suitable black pigment includes carbon black.

A specific example of a suitable white pigment includes titaniumdioxide.

Specific examples of suitable yellow pigments include diarylide yellowand diazo based pigments.

Specific examples of suitable blue pigments include phthalocyanine blue,basic dye pigments, and phthalocyanines, preferably, copper (II)phthalocyanine.

Specific examples of suitable red pigments include anthraquinone (red),napthole red, monoazo based pigments, quinacridone pigments,anthraquinone, and perylenes. Preferably, perylene red is used.

Specific examples of suitable green pigments include phthalocyaninegreen and nitroso based pigments. Specific examples of suitable orangepigments include monoazo and diazo based pigments, quinacridonepigments, anthraquinones and perylenes.

Specific examples of suitable violet pigments include quinacrinodeviolet, basic dye pigments and carbazole dioxazine based pigments.Preferably, quinacridone violet is used.

Suitable aqua, brown, gray, and pink colors can easily be formulated bycombining several pigments.

One skilled in the art is able to form any color as desired by combiningdifferent pigments.

The pigment can be present in the ink composition in an amount thatprovides coloration that is visible without magnification to facilitateidentification of the individual colored optical glass fiber. The amountof pigment referred to in the present specification refers to the amountof dry pigment.

The amount of the pigment should not be so great as to significantlyreduce the cure speed of the ink composition or result in otherundesirable effects.

Examples of suitable amounts of pigment have been found to be higherthan about 1% of the total weight of the composition. Generally, theamount is less than 25%, preferably less than about 15%, more preferablyless than about 10%, based on the total weight of the ink composition.

Preferred amounts of each pigment are from about 0.5 to about 15% byweight, more preferably from about 1 to about 10% by weight,particularly preferred, from about 3 to about 8% by weight.

Coated optical fibers are often used in ribbon assemblies. Because ofthe versatility of the presently invented in ink coating composition,this composition is very well suited for use on coated optical glassfibers in ribbon assemblies. A release agent (D) can thus advantageouslybe added to the ink coating to allow easy access to the individualfibers by separating the matrix material from the ink coating, thusimproving the so-called fiber break-out properties.

As a release agent (D) reactive or non-reactive silicone release agentscan be used, where reactive silicones comprise silicones having areactive group, e.g. an acrylate function, capable of reacting with thefunctional groups of the oligomers and/or monomers diluents forming thephoto-curable composition. Also non-reactive release agents can be used.Preferably, the silicone release agent is organo modified. Preferablypolymeric silicone release agents, such as polyether based siliconerelease agents. Component (D) can be present in amounts between about 1and about 9% by weight, preferably between about 1.5 and about 6% byweight, particularly preferred between about 2 and about 5% by weight.Examples of suitable non-reactive release agents are polydimethylsiloxane based silicone release agents. According to a preferredembodiment of the present invention, the amount of silicone releaseagent is sufficient to obtain good release properties but sufficientlylow to preclude failure in the 60° C. water soak test.

In particular, the Applicant has found that when the base oligomer issubstantially free from urethane acrylate compounds and comprises abisphenol A epoxy diacrylate or, preferably, a fatty acid modifiedbisphenol A epoxy acrylate or a mixture thereof, the above preferredrelease agents can be present in relative high amounts (up to 9% byweight) into the photo-curable composition, without negatively affectingthe final mechanical properties of the cured resin. On the other side,the Applicant has observed that if urethane acrylate oligomers arepresent in amounts higher than about 5% by weight in the photo-curablecomposition, said high amounts of release agent may result inundesirable de-mixing phenomena in the composition. Commercial examplesof non-reactive polymeric silicone release agents are CoatOSil 3500 andCoatOSil 3501, supplied by CK Witco, DC 57, DC 190, and DC 193, suppliedby Dow Corning, and Byk333 supplied by Byk. Other additives which can beused in the radiation-curable carrier system include, but are notlimited to, lubricants, wetting agents, antioxidants and stabilizers.The selection and use of such further additives is within the skill ofthe art.

The colored coating generally has a Tg of at least about 30° C., morepreferably at least 50° C.

Colored optical fibers according to the present invention can bemanufactured according to conventional manufacturing techniques. Thecolored coating compositions according to the present invention areparticularly suitable for being applied at high speed, e.g. at about1000 m/min and up to about 2000 m/min. For instance, colored opticalfibers according to the present invention can be produced by applyingthe colored coating layer at a line speed of about 1000 m/min using two300 w/inch, 10 inches length, 9 mm D type bulb lamps or at a line speedof about 1770 m/min using two 600 w/inch, 10 inches length, 11 mm D typebulb lamps.

When applied onto a primary coating system comprising an inner and anouter primary coating, the colored coating typically has a thickness offrom about 3 μm to about 10 μm. In general, the application of thecolored layer takes place within one month from the application of theprimary coating onto the optical fiber.

The so obtained optical fibers can be used as such for the manufactureof optical fiber cables or they may advantageously be used formanufacturing optical fiber ribbons, by edge-bonding or preferably byencasing the optical fibers into a matrix material.

Suitable matrix materials for manufacturing an optical fiber ribboncomprising a colored optical fiber according to the present inventionare those known in the art. The matrix material is generally obtained bycuring a radiation-curable composition comprising oligomers and monomershaving at least one functional group capable of polymerization whenexposed to actinic radiation. Suitable radiation-curable oligomers ormonomers are now well known and within the skill of the art. Commonly,the radiation-curable functionality used is ethylenic unsaturation,which can be polymerized preferably through radical polymerization.Suitable matrix materials. Examples of radiation-curable compositionssuitable for being applied as matrix material are disclosed, forinstance in U.S. Pat. Nos. 4,844,604, 5,881,194, and 5,908,873 which arehereby incorporated by reference. An example of a commercial matrixmaterial is Cablelite® 3287-9-53 (DSM Desotech).

Optical fiber ribbons can be manufactured according to conventionalribboning methods, which include the single stage process and the twostage process.

In the single stage process, also known as “tandem” process, theapplication of the colored layer and the ribboning of the colored fibertakes place on the same coating line. The colored coating compositionsaccording to the present invention are particularly suitable forproducing optical fiber ribbons according to this method.

Thus, the fibers forming the ribbon are first passed throughconventional colored coating applicators, and the colored layers aresimultaneously radiation cured, e.g. by passing the fibers through two300 w/inch, 10 inches length, 9 mm D type bulb lamps. The so coloredcoated optical fibers, disposed in parallel to each other, are thenpassed through the matrix material applicator and then the matrixmaterial is cured, e.g. by passing the so formed ribbon through two 300w/inch, 10 inches length, H type bulb lamps. The line speed of thetandem process is generally of about 250–300 m/min.

Alternatively, each single fiber is separately color coated in a firststage (at a speed of about 1000 or 1700 m/min, for instance) and coiledonto its relative coil bobbin. On a separate line, the fibers are thenunwound from the bobbin, disposed in parallel to each other and then thematrix material is applied, e.g. at a line speed of about 250–300 m/min,for forming the fiber ribbon

The time delay between ink and matrix application is generally fromabout 8 hours up to about 4–5 days.

The cure degree of a colored coating layer according to the invention ispreferably of at least 80%, preferably of at least 85%, more preferablyof at least 95%, said cure degree being determined by means ofMICRO-FTIR technique, by determining the amount of the unreactedacrylate unsaturations in the final cross-linked resin with respect tothe initial photo-curable composition.

A colored optical fiber according to the present invention comprises acolored layer which, when the fiber is in turn coated with a matrixmaterial, shows the desired optimized adhesion properties to theunderlying coating layer and to the matrix material.

As previously mentioned, the adhesion of the colored layer to theunderlying coating layer is sufficiently high as to avoid undesiredseparation of the color coded layer from the fiber, when handling of thefiber occurs. This property can be easily evaluated by means of manualtests, such as by cutting the colored layer, along the fiber's lengthwith a blade and then determining how easily the colored layer can bemanually separated from the underlying coating layer.

The adhesion of the colored layer to the matrix material should insteadbe adequately balanced in order to meet both the requirements of goodfiber break-out and of water soak resistance. Thus, on one side, saidadhesion of the colored layer to the matrix material should besufficiently low in order to allow an easy removal of the matrixmaterial from the colored fiber, without causing separation of thecolored layer. On the other side said adhesion to the matrix materialshould be sufficiently high for imparting the desired water soakproperties to the fiber in the ribbon.

Moreover, according to a preferred embodiment of the present invention,the above desirable properties of the colored layer are adequatelybalanced with the requirement of achieving an acceptable solventresistance, as measured by the MEK resistance. In order to achieve, forexample, a desired MEK resistance, the radiation curable colored coatingcomposition of the present invention preferably comprises a mixture offrom about 2.4:1 to about 2.2:1, more preferred, about 2.3:1 ofunmodified to modified bisphenol A epoxy diacrylate (A), and a ratio ofabout 1.2:1 of the alkoxylated aliphatic glycol diacrylate diluent (B1)to the trifunctional acrylate diluent (B2), and components (C) to (E).Said composition shows a preferable MEK resistance of at least about 110double rubs, preferably, at least about 115, more preferred, at leastabout 120, particularly preferred, at least about 130, most preferred atleast about 150 MEK double rubs.

The MEK double rub test is performed on a coated and colored opticalfiber as follows. A coated optical fiber is colored with the coloredcoating composition of the present invention on an OFC 52 apparatus ofNextrom at a line speed of 1000 m/min under one D type bulb lamp at 100%capacity under a nitrogen purge of 40 liter per minute. Approximately 1meter of the colored coated optical fiber is then fixed firmly onto atable with 4 pieces of tape of approximately 5 cm width (such as, forexample, Scotch tape) so as to divide the fiber into three parts ofapproximately 15 cm. The MEK test is then performed on said three partsof the fiber (three samples per fiber). The amount of MEK double rubs isthen measured as follows: a Texwipe TX 404T wipe is folded in quartersto form a folded pad, said pad is placed over the round edge of a 16ounces (453.6 gram) hammer and secured with a small rubber band. A fewdrops of methyl ethyl ketone (MEK), reagent grade, are added to the padand allowed to disperse, but not allowed to dry. The pad secured to thehammer is placed on the coated and colored fiber sample keeping thehammer's handle level and then moved approximately 6 inches (15.24 cm)along the sample keeping the handle of the hammer as level as possibleand then bringing the hammer back to its starting position with thehandle as level as possible. The keeping the hammer's handle levelmaintains a fairly consistent weight on the sample. Said action isconsidered one cycle, corresponding to two single rubs. A coloredcoating composition is said to pass if it survives a minimum of 100 MEKdouble rubs,

The following examples are given as particular embodiments of theinvention and to demonstrate the practice and advantages thereof. Theexamples are given by way of illustration and are not intended to limitthe specification or claims.

EXAMPLE 1

Preparation of Photocurable Colored Composition

An ink composition (of blue and red color respectively) was prepared bymixing the components of Table 1.

TABLE 1 Photo curable colored composition Blue Red Components of inkcomposition (wt. %) (wt. %) Bisphenol A epoxy diacrylate (Mw = 524)24.17 21.38 Fatty acid modified Bisphenol A epoxy 26.10 23.06 diacrylate(Mw = 500) Propoxylated neopentyl glycol diacrylate 23.69 21.04 (Mw =328) TMPTA⁽¹⁾ 8.95 15.59 2,4-di-tert-butyl-p-cresol (BTH) 0.49 0.43Irgacure 819⁽²⁾ 0.97 0.87 Irgacure 907⁽³⁾ 2.90 2.57 Darocure 1173⁽⁴⁾3.86 3.43 Benzophenone 1.93 1.71 CoatOSil 3500⁽⁵⁾ 3.30 3.30 CoatOSil3501⁽⁶⁾ 1.00 1.00 Pigment blue: copper(II)phthalocyanine⁽⁷⁾ 1.04 —Pigment white: rutile titanium dioxide⁽⁷⁾ 1.60 3.82 Pigment red:perylene red⁽⁷⁾ — 1.33 Pigment violet: quinacridone violet⁽⁷⁾ — 0.48⁽¹⁾TMPTA is trimethylol propane triacrylate ⁽²⁾Irgacure 819 isbis-(2,4,6-trimethylbenzoyl)-phenyl-phosphine oxide (supplied by CibaGeigy) ⁽³⁾Irgacure 907 is2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-propan-1-one (supplied byCiba Geigy) ⁽⁴⁾Darocure 1173 is 2-hydroxy-2-methyl-1-phenyl-propan-1-one(supplied by Ciba Geigy) ⁽⁵⁾CoatOSil 3500 is a polydimethyl siloxanebased silicone release agent (CK Witco) ⁽⁶⁾CoatOSil 3501 is apolydimethyl siloxane based silicone release agent (CK Witco) ⁽⁷⁾amountdry pigment as present in a pigment dispersion

EXAMPLE 2

Preparation of Photocurable Colored Composition

An ink composition similar to the composition of Table 1 was preparedcontaining the same total amount of release agent, being a reactivesilicone release agent.

EXAMPLE 3

Preparation of Photo Curable Colored Composition

An ink composition similar to the composition of Table 1 was prepared byreplacing the photoinitiator package of the composition of Table 1 by a5–6% by weight mixture of Irgacure 819 and Darocure 1173 depending onthe color used.

The amount of the other components was further adjusted to add up to atotal amount of 100% by weight.

EXAMPLE 4

Preparation of Photo Curable Colored Composition

An ink composition similar to the composition of Table 1 was prepared byreplacing the fatty acid modified bisphenol A epoxy diacrylate with thesame amount of the bisphenol A epoxy diacrylate indicated in said table,containing as the only release agent 1% by weight of a polydimethylsiloxane based silicone release agent, and having some N-vinylcaprolactam as a diluent.

The amount of the other components was further adjusted to add up to atotal amount of 100% by weight.

EXAMPLE 5

Preparation of Photo Curable Colored Composition

An ink composition similar to the composition of Table 1 was prepared byreplacing the release agents by 1% by weight of CoatOSil 3501. Theamount of the other components was further adjusted to add up to a totalamount of 100% by weight.

Comparative Experiment A

An ink composition was prepared by mixing the components of Table 2.

TABLE 2 Photo curable colored composition Wt. % of total Components ofink composition composition Ebecryl 3700 (epoxy acrylate) 51.0Alkoxylated aliphatic diacrylate 30.0 TMPTA 9.0 Irgacure 819 1.0Irgacure 907 3.0 Benzophenone 4.0 Blue pigment (Penn Color) 1.5 BYK333⁽⁸⁾ 0.5 ⁽⁸⁾BYK 333 is a polyether modified dimethyl polysiloxane.Comparative Experiment B

A commercial ink composition comprising a urethane acrylate oligomer wasused.

EXAMPLE 6

Determination of Adhesion of the Colored Layer to the Fiber and CureDegree at Different Application Speeds

The above colored photo curable compositions where applied with athickness of about 5–6 μm onto Corning® SMF-28™ CPC6 optical fibers.

The application of the ink compositions has been made at differentspeeds, in order to evaluate the effects of the cure speed onto theproperties of the optical fibers.

In particular, the inks have been applied onto the optical fibers at aspeed of 100 m/min, 250 m/min, 500 m/min or 1000 m/min using a coloringline equipped with two 300 w/inch, 10 inches length , 9 mm UV D typebulb lamps.

The curing % of each ink was measured by means of FTIR, by determiningthe % amount of RAU (reacted acrylate unsaturation), according to themethodology described in WO 98/50317.

The adhesion of the ink to the fibers was determined by cutting the inklayer with a knife and evaluating how easily the ink layer could beseparated from the underlying coating layers by acting on the cuts'edges with the knife's blade.

The following table 3 illustrates the results of the ink adhesion testand the curing % of the tested ink compositions. In the table, theacronym “VG” means a very good degree of ink adhesion to the fiber while“G” means good adhesion, thus indicating that the ink can not beseparated by the underlying layer or only small flakes can be separated,respectively. Symbol “A” means an acceptable adhesion, thus indicatingthat the ink layer is sufficiently adherent to the underlying coatinglayer for the purposes of handling the fiber without undesiredseparation of the colored layer, said colored layer being neverthelessseparable from the underlying coating by acting on the interface of thetwo layers with the knife's blade. Symbols “P” and “VP” means pooradhesion and very poor adhesion, respectively.

TABLE 3 Cure degree % of the colored layer (1) and adhesion of thecolored layer to the underlying coating layer (2) at different curingspeeds Cure degree (1) and ink adhesion to fiber (2) 100 m/min 250 m/min500 m/min 1000 m/min (1) (2) (1) (2) (1) (2) (1) (2) Ex. 1 — — ≧95 VG≧95 VG ≧85 G Ex. 2 — — ≧95 VG — — — — Ex. 3 — — ≧95 VG — — — — Ex. 4 — —≧95 G ≧95 G — — Ex. 5 — — ≧95 VG ≧95 VG ≧85 A Comp. A ≧95 VG/G ≧95 G/A —— — — Comp. B — — — — ≧80 A/P <80 P

EXAMPLE 7

Determination of Water Soak Resistance on Single Fibers

The colored optical fibers manufactured according to Example 6 weretested to determine the respective water soak resistance, by measuringthe variation of the attenuation of the signal transmitted through theoptical fiber immersed into water at a temperature of 60° C.

1000 m of each single fiber were thus loose coiled into (coils of about300 mm diameter), and immersed in a thermostatic vessel containing tapwater at 60° C.

The optical attenuation of the fiber has been measured at 1550 nm withthe back-scattering technique, using an ANRITSU mod. MW 9005C OTDR(optical time domain reflectometer). The measurements were performedeach 30 minutes for the first ten days, then daily up to the end of thefirst month and then weekly.

Increases of less than 0.05 db/km were measured for all the testedfibers after 120 days aging in water.

EXAMPLE 8

Determination of the Fiber Break-out Properties of the Colored Layer

Optical fiber ribbons containing optical fibers having a colored layeras indicated in Example 6 were manufactured using Cablelite® 3287-9-53(DSM Desotech) as the matrix material composition according to thefollowing manufacturing techniques, previously described:

-   -   Tandem process (colored layer and matrix material applied both        at either 100 or 250 m/min)    -   Two step process (colored layer applied at 1000 m/min; matrix        material at 250 m/min), with a delay of about one day between        the two applications.

The total thickness of the ribbon was of about 300 μm.

The ribbons manufactured according to the “tandem” process containedfour optical fibers each (two with red pigment and two with bluepigment). Ribbons manufactured according to the two stage processcontained 6 colored optical fibers. The determination of the fiberbreak-out properties has been made by manually opening the ribbon andevaluating how easily the fiber could be separated from each other andfrom the ribbon matrix, on a “pass/fail” basis test. A “pass” rate hasthus been assigned to those fibers showing a regular fiber separationfor at least about 500 mm and a removal length of the matrix material ofabout 50 mm, without any matrix material being left onto the fiber orany ink removal from the fiber. A “fail” rate has been assigned to thosefiber not satisfying the above conditions. The results are reported inthe following table 4.

TABLE 4 Fiber break-out Fiber break out at Tandem process Two-step INK100 m/min 250 m/min 1000 + 250 m/min Ex. 1 — Pass Pass Ex. 2 — Pass —Ex. 3 — Pass — Ex. 4 — Pass Pass Comp. A Pass Fail — Comp. B — Pass Pass

EXAMPLE 9

Determination of Water Soak Performances on the Ribbonized Fibers

1000 m of ribbons manufactured according to Example 8 were tested fordetermine the water soak resistance of colored optical fibers, bymeasuring the variation of the attenuation of the signal transmittedthrough the optical fiber immersed into water at a temperature of 60°C., according to the methodology described in Example 7.

Ribbons manufactured with the matrix material mentioned in Example 8 areidentified as MM1 in the following table 6.

In addition, a second set of ribbons has been manufactured using analternative matrix material, identified in Table 6 as MM2, having thecomposition as given in Table 5 (expressed as % by weight ofcomponents):

TABLE 5 Matrix composition MM2 Components of matrix composition MM2 Wt.% HEA-IPDI-propoxylated (n = 4) BPA-IPDI-HEA⁽¹⁾ 17.92HEA-IPDI-Priplast3192-IPDI-HEA⁽²⁾ 29.2 Isobornyl acrylate 29.8Ethoxylated (n = 3) trimethylol propane 1.55 triacrylate Trimethylolpropane triacrylate (TMPTA) 18.5 Lucerin TPO⁽³⁾ 1.5 Irgacure 184 1.5Irganox 1010⁽⁴⁾ 0.03 ⁽¹⁾HEA = hydroxy ethyl acrylate; IPDI = isophoronediisocyanate; propoxylated (n = 4) bisphenol A ⁽²⁾Priplast 3192: dimeracid modified hexanediol diacrylate ⁽³⁾Lucerin TPO:2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide (supplied by BASF)⁽⁴⁾Irganox 1010:penta-erithrityl-tetrakis-(3-(3,5-di-tert-butyl-4-hydroxy-phenyl)-propionate(manufactured by Ciba Specialty Chemicals Co.)

Fibers showing an increase in the attenuation value of 0.05 db/km ormore after less than two weeks of testing are considered not acceptable.Fibers showing an increase in the attenuation value of less than about0.05 db/km after at least two weeks of testing are consideredacceptable. Fibers showing an increase in the attenuation value of lessthan about 0.05 db/km after at least one month of testing are preferred.Fibers showing an increase in the attenuation value of less than about0.05 db/km after two months or more are particularly preferred. Mostpreferred are those fiber for which an increase in the attenuation valueof less than about 0.05 db/km is measured for at least 120 days,corresponding to the maximum run time of the test.

Table 6 shows the results of these tests on ribbons manufactured byusing the tandem manufacturing technique at 250 m/min or by using thetwo step manufacturing technique (application of colored layer at 1000m/min and of matrix material at 250 m/min).

The value reported in table 6 refers to the day on which the first fiberof the ribbon shows (apart variations due to experimental errors) anincrease of the attenuation value of more than 0.05 db/km, with respectto the initial attenuation value of the same optical fiber.

TABLE 6 Water soak test for ribbons No. of days with variation ofattenuation of less than 0.05 db/km for ribbons manufactured at 250m/min (tandem) 1000 + 250 m/min with MM1 with MM2 with MM1 Ex. 1 70 4780 >120 52 120 Ex. 2 — 40 — Ex. 3 — 80 — Ex. 4 30 20 92 70 54 >120 Comp.B 2 3 8 3 4

1. An optical fiber comprising a radiation curable internal coating anda radiation curable colored coating disposed to surround said internalcoating, wherein, when said fiber is coated with a radiation curablematrix material and assembled into an optical fiber ribbon: said colorcoating has a degree of adhesion to the inner coating which is higherthan the degree of adhesion to the matrix material; and said opticalfiber assembled into said optical fiber ribbon shows, upon aging for atleast two weeks in water at 60° C., an increase in the attenuation ofthe transmitted signal at 1550 nm of less than 0.05 db/km with respectto the attenuation of the assembled optical fiber measured before aging.2. The optical fiber according to claim 1, wherein the increase in theattenuation of the transmitted signal is less than about 0.05 db/km,upon aging of the assembled fiber for at least one month in water at 60°C.
 3. The optical fiber according to claim 1, wherein the increase inthe attenuation of the transmitted signal is less than about 0.05 db/km,upon aging of the assembled fiber for at least two months in water at60° C.
 4. The optical fiber according to claim 1, wherein said internalcoating comprises an inner primary coating and an outer primary coatingand the colored coating has a thickness of about 3 to about 10 microns.5. An optical fiber ribbon comprising a plurality of optical fibersbound together by a radiation curable matrix material, said fiberscomprising a radiation curable internal coating layer and a radiationcurable colored coating layer disposed to surround said internalcoating, wherein said colored coating layer has a degree of adhesion tothe internal coating which is higher than the degree of adhesion to thematrix material, said degree of adhesion to the matrix material beingsufficiently high such that said optical fibers show, upon aging for atleast two weeks in water at 60° C., an increase in the attenuation ofthe transmitted signal at 1550 nm of less than 0.05 db/km with respectto the attenuation of the optical fibers measured before aging.
 6. Anoptical fiber comprising a radiation curable internal coating and aradiation curable colored coating disposed to surround said internalcoating wherein said colored coating comprises: (A) 40–60% by weight ofa bisphenol A epoxy diacrylate, a modified bisphenol A epoxy diacrylateor a mixture of both; (B1) 15–30% by weight of an alkoxylated aliphaticglycol diacrylate diluent; (B2) 5–25% by weight of trifunctionalacrylate diluent; (C) 6–20% by weight of a photoinitiator systemconsisting of less than 4% by weight of benzophenone and at least twodifferent homolytic free-radical photoinitiators; (D) 1–9% by weight ofa polydimethylsiloxane based silicone release agent; and (E) 1–15% byweight of a dry pigment; said composition comprising less than 5% byweight of a urethane acrylate, whereby, when said fiber is coated with aradiation curable matrix material and assembled into an optical fiberribbon, said optical fiber shows, upon aging for at least two weeks inwater at 60° C., an increase in the attenuation of the transmittedsignal at 1550 nm of less than 0.05 db/km with respect to theattenuation of the assembled optical fiber measured before aging.
 7. Anoptical fiber according to claim 6, wherein the two homolyticphotoinitiators of component (C) of the radiation curable coloredcoating composition differ in their respective photosensitivity.
 8. Theoptical fiber according to claim 6, wherein the radiation curablecolored coating composition further comprises less than 3% by weight ofN-vinyl caprolactam.
 9. The optical fiber according to claim 6, whereinthe radiation curable colored coating composition comprises as thetrifunctional acrylate diluent (B2) trimethylol propane triacrylate. 10.The optical fiber according to any one of claims 1–9, wherein theradiation curable colored coating composition consists essentially of:(A) 40–60% by weight of a bisphenol A epoxy diacrylate, a modifiedbisphenol A epoxy diacrylate or a mixture of both; (B1) 15–30% by weightof an alkoxylated aliphatic glycol diacrylate diluent; (B2) 5–25% byweight of trifunctional trimethylol propane triacrylate; (C) 6–20% byweight of a photoinitiator system consisting of less than 4% by weightof benzophenone and at least two homolytic free-radical photoinitiators;(D) 1–9% by weight of a polydimethylsiloxane based silicone releaseagent; and (E) 1–15% by weight of a dry pigment.
 11. The optical fiberaccording to claim 10, wherein the alkoxylated aliphatic glycoldiacrylate diluent (B1) of said radiation curable colored coatingcomposition is ethoxylated aliphatic glycol diacrylate.
 12. The opticalfiber according to claim 10, wherein component (D) of said radiationcurable colored coating composition is a non-reactivepolydirnethylsiloxane based silicone release agent.